This invention pertains generally to a system for measuring line or dot spaces in a periodic matrix on a surface and, particularly, to a system for measuring space width or dot diameter on a cathode-ray tube faceplate panel.
During the production of cathode-ray tubes for color television receivers, a black matrix is applied to the inside surface of the faceplate panels. The black matrix consists of parallel lines which extend vertically as defined by the viewing orientation of a conventional tube. Black lines are spaced at desired intervals leaving transparent glass in the spaces between the matrix lines. With a high-resolution display tube, the black matrix is applied with transparent dot spaces in the matrix. The transparent spaces are coated with slurries of materials containing phosphors which emit the three primary colors of light , i.e., red, green and blue, when impacted by electrons. The three phosphors are alternately applied in a repetitive sequence such as red, green and blue to all the transparent spaces of the panel. Prior to the application of the phosphors, it is desirable to measure the space widths formed by the transparent spaces on standard panels and measure the transparent dot diameters on high-resolution display panels to verify that they are within acceptable dimensional tolerances in order to avoid the expensive application of phosphors to improperly matrixed faceplate panels.
In order to measure the dot diameters and space widths of each panel, the faceplate panel is placed between a stationary light source and a detector. Light is passed through the spaces within the matrix to form an image on the light detector. This image is scanned in a direction substantially perpendicular to the matrix lines, and the variation in light caused by the opaque areas and the transparent spaces is sensed by the detector and provided to a measuring system. The panel is moved to various positions and the scanning and measuring repeated.
A system for carrying out the above operation is shown in U.S. Pat. No. 4,525,735. A problem with such a system is that the panel movement takes a relatively long time. This lowers the production rate to a point where it may be impossible to meet a particular desired inspection time, e.g., 15 seconds per panel. Another problem with this system is that invalid readings can be processed, which can result in the acceptance of an out-of-tolerance panel. Yet another problem is that the correct readings are fixed by the circuitry. Thus, different types of panels or shadow mask screens, e.g., normal resolution for regular television or high resolution for data display terminals, cannot be inspected by the same system.